Technical Field
The present invention relates to a terminal apparatus and a communication method thereof.
Description of the Related Art
In the uplink of the 3rd Generation Partnership Project Long Term Evolution (3GPP LTE), single carrier transmission is performed to maintain a low cubic metric (CM). More specifically, in the presence of data signals, the data signals and control information are time multiplexed and transmitted in a physical uplink shared channel (PUSCH). The control information includes response signals (positive/negative acknowledgments (ACK/NACK), hereinafter called “ACK/NACK signals”) and channel quality indicators (hereinafter called the “CQIs”). Data signals are divided into code blocks (CB), and a cyclic redundancy check (CRC) code is added to each code block for error correction.
ACK/NACK signals and CQIs have different allocation methods. (See Non-Patent Literatures 1 and 2, for example). More specifically, ACK/NACK signals are allocated in parts of a data signal resource by puncturing parts of the data signals (4 symbols) mapped to the resource adjacent to Reference Signals (RSs) (i.e., overwriting the data signals with the ACK/NACK signals). In contrasts, CQIs are allocated over entire sub-frames (2 slots). Since the data signals are allocated in resources other than the CQI allocated resource, no CQIs are punctured (see FIG. 1.) The reasons for the difference in allocation are as follows: the allocation or non-allocation of an ACK/NACK signal depends on the presence or absence of data signals in downlink. In other words, it is more difficult to predict the occurrence of ACK/NACK signals than it is to predict that of CQIs; hence, puncturing capable of allocating the resource of a suddenly occurring ACK/NACK signal is used during mapping of ACK/NACK signals. Meanwhile, the timing of CQI transmission (i.e., sub-frames) is predetermined based on notification information, which allows the determination of allocation of data signal and CQI resources. Since ACK/NACK signals are important information, they are assigned to symbols in the vicinity of pilot signals, which have high estimation accuracy of transmission paths, thereby reducing ACK/NACK signal errors.
A modulation and coding rate scheme (MCS) for data signals in uplink is determined by a base station apparatus (hereinafter called the “base station” or “eNB”) based on the channel quality of the uplink. An MCS for control information in the uplink is determined by adding an offset to the MCS for data signals (see Non-Patent Literature 1, for example). More specifically, since control information is more important than data signals, the MCS for control information is set to a lower transmission rate than the MCS for data signals. This guarantees high-quality transmission of control information.
For example, in the 3GPP LTE uplink, if control information is transmitted in a PUSCH, the amount of resource assigned to the control information is determined based on a coding rate indicated in the MCS for data signals. More specifically, as shown in equation 1 below, the amount of the resource Q assigned to the control information is obtained by multiplying the inverse of the coding rate of data signal by an offset.
      (          Equation      ⁢                          ⁢      1        )                                Q          =                      ⌈                                                            (                                      O                    +                    P                                    )                                ·                                  M                  sc                                      PUSCH                    ⁢                                          -                                        ⁢                    initial                                                  ·                                  N                  symb                                      PUSCH                    ⁢                                          -                                        ⁢                    initial                                                  ·                                  β                  offset                  PUSCH                                                                              ∑                                      r                    =                    0                                                        C                    -                    1                                                  ⁢                                  K                  r                                                      ⌉                                                [          1          ]                    
With reference to equation 1, O indicates the number of bits in control information (i.e., ACK/NACK signal or CQI) and P indicates the number of bits for error correction added to the control information (for example, the number of bits in CRC and in some cases, P=0). The total of O and P (O+P) indicates the number of bits in uplink control information (UCI). MSCPUSCH-initial, NSymbPUSCH-initial, C and Kr indicate the transmission bandwidth for PUSCH, the number of symbols transmitted in the PUSCH per unit transmission bandwidth, the number of code blocks into which data signals are divided, and the number of bits in each code block, respectively. UCI (i.e., control information) includes ACK/NACK, CQI, a rank indicator (RI), which indicates rank information, and a precoding matrix indicator (PMI), which provides precoding information.
With reference to equation 1, (MSCPUSCH-initial·NSymbPUSCH-initial) indicates the amount of transmission data signal resources, ΣKr indicates the number of bits in a single data signal (i.e., the total number of bits in code blocks into which the data signal is divided). Accordingly, ΣKr/(MSCPUSCH-initial·NSymbPUSCH-initial) represents a value that depends the coding rate of the data signal (hereinafter, called “coding rate”). The (MSCPUSCH-initial·NSymbPUSCH-initial)/ΣKr shown in equation 1 indicates the inverse of the coding rate of data signal (i.e., the number of resource elements (RE: resource composed of one symbol or one sub-carrier) used to transmit one bit) βoffsetPUSCH indicates the amount of offset by which the above-mentioned inverse of the coding rate of data signal is multiplied, and is reported from a base station to each terminal apparatus (hereinafter, called the “terminal” or UE) via upper layers. More specifically, a table indicating candidates of the amounts of offset βoffsetPUSCH is defined for each part of control information (i.e., ACK/NACK signal and CQI). For example, a base station selects one amount of offset βoffsetPUSCH from the table (for example, see FIG. 2) containing candidates for the amount of offset βoffsetPUSCH defined for ACK/NACK signal and then notifies a terminal of a notification index corresponding to the selected amount of offset. As is evident from the term “PUSCH-initial,” (MSCPUSCH-initial·NSymbPUSCH-initial) represents the amount of transmission resource for the initial transmission of a data signal.
The standardization of 3GPP LTE-Advanced, which provides higher-speed transmission than 3GPP LTE, has started. The 3GPP LTE-Advanced system (hereinafter, may be called “LTE-A system”) follows the 3GPP LTE system (hereinafter, called “LTE system”). In 3GPP LTE-Advanced, base stations and terminals that can communicate in a wideband frequency range of 40 MHz or higher will be introduced to achieve downlink transmission rates of up to 1 Gbps.
In an LTE-Advanced uplink, the use of single user multiple input multiple output (SU-MIMO) transmission in which a single terminal transmits data signals in a plurality of layers has been studied. In the SU-MIMO communications, data signals are generated in a plurality of code words (CWs), each of which is transmitted in different layers. For example, CW#0 is transmitted in layers #0 and #1, and CW#1 is transmitted in layers #2 and #3. In each CW, a data signal is divided into a plurality of code blocks and CRC is added to each code block for error correction. For example, a data signal in CW#0 is divided into five code blocks and a data signal in CW#1 into eight code blocks. The “code word” can be regarded as a unit of data signals to be retransmitted. The “layer” is a synonym of a stream.
Unlike the above-mentioned LTE-A system, the LTE systems disclosed in the above-mentioned Non-Patent Literatures 1 and 2 assume the use of the non-MIMO transmission in uplink. In the non-MIMO transmission, a single layer is used at each terminal.
In the SU-MIMO transmission, control information is transmitted in a plurality of layers in some cases, and it is transmitted in one of the plurality of layers in other cases. For example, in an LTE-Advanced uplink, allocation of an ACK/NACK signal in a plurality of CWs and of a CQI in a single CW has been studied. More specifically, since an ACK/NACK signal is the most important information in all parts of control information, the same ACK/NACK signal is allocated in all the CWs (i.e., the same information is assigned to all layers (rank-1 transmission)), thereby reducing inter-layer interference. The same ACK/NACK signals transmitted in a plurality of CWs (i.e., space-division multiplexed) are combined into a single part of information on a transmission path, thereby eliminating the need for the receiving side (base station) to separate the ACK/NACK signals transmitted in a plurality of CWs. Accordingly, inter-layer interference that may occur on the receiving side during the separation does not occur. Thus, high receiving quality can be achieved. Note that the description below assumes that the control information is an ACK/NACK signal and allocated in two CWs (CW#0 and CW#1).